Graphene is an atomically thick, two dimensional sheet composed of sp2 carbons in a honeycomb structure. It can be viewed as the building block for all the other graphitic carbon allotropes. Graphite (3-D) is made by stacking several layers of graphene on top of each other, with an interlayer spacing of ˜3.4 Å and carbon nanotubes (1-D) are a graphene tube.
Single-layer graphene is one of the strongest materials ever measured, with a tensile strength of ˜130 GPa and possesses a Young's modulus of ˜1 TPa. Graphene's theoretical surface area is ˜2630 m2/g and the layers are gas impermeable. It has very high thermal (5000+W/mK) and electrical conductivities (up to 6000 S/cm). The observed superior properties of graphene introduced it as a potential candidate material for many applications including but not limited to:
(a) additive for mechanical, electrical, thermal, barrier and fire resistant properties of a polymer;
(b) surface area component of an electrode for applications such as fuel cells, super-capacitors and lithium ion batteries;
(c) conductive, transparent coating for the replacement of indium tin oxide; and
(d) components in electronics.
Graphene was first reported in 2004, following its isolation by Professor Geim's group at The University of Manchester. Graphene research since then has increased rapidly. Much of the “graphene” literature is not on true monolayer graphene but rather two closely related structures:
(i) “few layer graphene”, which is typically 2 to 10 graphene layers thick. The unique properties of graphene are lost as more layers are added to the monolayer and at 10 layers the material becomes effectively bulk graphite; and
(ii) Graphene oxide (GO), which is a graphene layer which has been heavily oxidised in the exfoliation process used to make it and has typically 30 at % (atomic percentage) oxygen content. This material has inferior mechanical properties, poor electrical conductivity and is hydrophilic (hence a poor water barrier).
There are a variety of methods to produce graphene [Ruoff 2009]. Novoselov et al. produced their first flakes by the mechanical exfoliation of graphite by using an adhesive tape to isolate individual layers [Novoselov 2004]. It has been shown subsequently that graphite can also be exfoliated by using ultrasonic energy to separate the layers when in an appropriate solvent, such as NMP (N-methyl pyrrolidone) [Coleman 2008 & 2009].
Graphite is an allotrope of carbon, the structure of which consists of graphene layers stacked along the c-axis in a staggered array usually denoted as ABAB. The layers are held together by weak van der Waals forces so that the separation between layers is 0.335 nm. Graphite is a cheap and abundant natural material, which makes it an excellent raw material for inexpensive production of graphene.
As noted above, graphite has been used to make graphene via exfoliation, wherein the stacked layers of graphite are separated to produce graphene. This has been achieved by using ultrasound (ultrasonic exfoliation, USE) and also by intercalating compounds into the graphite interlayer structure so as to weaken the interlayer bonding and promote layer separation.
There are two routes that have been reported to intercalate compounds into graphite structure: chemical and electrochemical. The chemical method is based on the direct reaction of solid graphite materials with the intercalation species (usually in liquid or vapour phase). This process is kinetically slow and usually assisted by sonication or heating. The second route, the electrochemical approach, involves generating the intercalated species through an electrochemical reaction on a graphite cathode or on a graphite anode.
The most famous example of the electrochemical approach is based on the lithium ion battery. For decades, graphite was used as negative electrode in a lithium ion battery due to its high electrical conductivity and its ability to host lithium between the graphene layers. The lithium-graphite intercalation compounds decompose readily in water giving rise to lithium hydroxide and free standing graphene sheets. Loh et al. mimicked the lithium ion battery principle to intercalate Li into graphite and then applied a sonication step to exfoliate graphite [US 2013/0102084 A1, and WO 2011/162727]. This work is also discussed in a related paper [Wang 2011]. However, due to the slow kinetic nature of the intercalation process, the lithium was limited to the areas close to the edges. Upon exfoliation in water, graphite with expanded edges was produced and further intercalation, water decomposition and sonication steeps were needed to achieve exfoliation.
Liu et al. [Liu 2008] reported the exfoliation of graphite using an ionic liquid-water mixture electrolyte to form “kind of IL-functionalized” graphene nanosheets. Scheme 1 in this paper suggests that the material was produced by the exfoliation of the anode but in their discussion the authors mention the role of the cation. Lu subsequently studied the route in more detail and discussed the possible mechanism involved in the production process [Lu 2009]. In their paper, they stated “according to the proposed mechanism by Liu, the positively charged imidazolium ion is reduced at the cathode to form the imidazolium free radical which can insert into the bonds of the graphene plane. At the fundamental level, there are several questionable aspects about the radical-insertion mechanism proposed by Liu, especially when the ILs are mixed with water at 1:1 ratio and where an operational voltage as high as 15 V is applied”. Lu et al. showed that the graphene nanosheet production is exclusively at the anode and is due to an interaction of decomposed water species and the anions from the ionic liquid, such as BF4.
The present inventors reported in WO2012120264-A1 the exfoliation of graphite through the electrochemical ammonium-graphite intercalated compound. Without sonication or repeating the intercalation/decomposition steps, the product was few layer graphene with a particle size in the submicron level. Swager and Zhong [Zhong 2012] proposed a method to intercalate graphite with Li and then with ammonia in two separate steps. However, due to the expanding nature of the cathode, the electrodes distance was initially large and hence high voltage was applied to overcome the high internal resistance (IR) drop. As a result, the organic solvent used as electrolyte dissociated at later stages of the process and hindered the intercalation process. Therefore, an additional sonication step was necessary to achieve reasonable exfoliation.
Huang et al [Huang 2012] have used molten LiOH at 600° C. to generate intercalated LixCy species via an in-situ reduction process. Huang reports that it is the reduced LixCy species (and not the Li ions) that causes the desired expansion of the graphite. The expanded graphite is subsequently exfoliated in a distinct, separate aqueous sonication step.
For completeness it is noted that under the right conditions the fragments from the disintegrated negative electrode can be nanoscale forms of a carbon. The production of carbon nanotubes from the intercalation of alkali metal ions into a graphite cathode has been reported by Kinloch et al. [Kinloch 2003]. These nanotubes were produced using a molten alkali halide electrolyte at high temperatures (600° C. or higher). In addition to carbon nanotubes, graphite platelets, carbon nanoparticles and amorphous structures were observed. However, no graphene was reported.
As is clear from the discussion above, a disadvantage of reported methods is that they produce a mixture of materials dispersed in solution (centrifugation is needed for separation). Furthermore, desirable yields of monolayer samples can only be achieved with prolonged application of ultrasonic exfoliation (USE) meaning that the lateral flake dimensions are very small (<1 micron), thus precluding many applications in electronic devices. Furthermore, the large-scale use of power ultrasound has raised safety concerns amongst industrial parties.
Another electrochemical method has been introduced, in WO201/132261, whereby double intercalation of graphite occurs with metal and organic ions. This method does not use any sonication, but it can suffer from decomposition of the organic solvent depending on the conditions used.
A yet further electrochemical method, in WO2015/019093, uses solvent-free ionic electrolytes where the electrolyte is selected from (i) an ionic liquid; (ii) a deep eutectic solvent; and (iii) a solid ionic conductor.
Intercalation compounds can also be produced by introducing a metal through the vapour phase and then reacting these ions. The layers of the intercalation compound can then be separated by stirring in an appropriate solvent, such as NMP [Valles 2008]. An intercalation approach has also been taken to separate graphene oxide aggregates by electrostatically attracting tetrabutylammonium cations in between the layers of the graphene oxide [Ang 2009]. This technique relies on the charges present in graphene oxide to attract the tetrabutylammonium cations.
Wang et al. have shown that ionic liquids are also appropriate solvents for ultrasonic exfoliation because of their stabilising effect on the resultant graphene. In this case, they mixed graphite powder with ionic liquids such as 1-butyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide ([Bmim][Tf2N]) and then subjected the mixture to tip ultrasonication for a total of 60 minutes using 5-10 minute cycles. The resultant mixture was then centrifuged [Wang 2010].
Graphene can also be produced by chemical vapour deposition. For example, methane can be passed over copper [Bae 2010]. Similar methods are also used to form carbon nanotubes [Simate 2010]. However, these methods are typically procedurally complex, require extremely high temperatures (e.g. up to 1000° C.) and usually require elaborate isolation techniques to obtain the material formed.
Alternatively silicon carbide can be decomposed to make a graphene film.
As is clear from the above comments, further methods for the production of graphene/graphite nanoplatelet structures are desired so as to mitigate or obviate the problems identified above. In particular, methods that produce graphene sheets with a controlled number of layers and flake size.
Advantageously, the methods should be scalable to allow for the production of graphene on a large scale. For instance, there is a desire to provide new methods that produce graphene/graphite nanoplatelet structures selectively over other carbon allotropes, which are amenable to scale-up to an industrial platform, which are more efficient, reliable, environmentally friendly, energy efficient, provide higher quality material, provide increased yields of material, provide larger sheets of material, provide easier isolation of material and/or which are procedurally simpler and/or cheaper than the methods of the prior art.